1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for imaging a fluid such as a bloodstream.
2. Description of the Related Art
A phase contrast method separates a fluid from a stationary tissue on the principle that the phase of a spin moving along a magnetic gradient shifts. In this method, as illustrated in FIGS. 1A, 1B, and 1C, two different echo data are obtained by applying .+-.A flow encode pulses M1 and M2 to a fluid in the A-direction, and a velocity image is reconstructed from the data as a result of the complex subtraction of the two echo data.
As another method of imaging a fluid, a method using stimulated echoes and "z-storage" radio-frequency (RF) pulses is introduced in "Magnetic Resonance in Medicine", Vol. 23, pp. 138-153 (1992). A flow encode pulse is a bipolar pulse. In this method, to enlarge a low-velocity region which can be imaged, the phase state of a spin after the first unipolar pulse is stored as longitudinal magnetization having a time constant which is generally an order of magnitude longer than that of transverse magnetization by using an RF pulse with a flip angle of 90.degree., thereby making it possible to extend the time length between the bipolar pulses. Additionally, with respect to transverse magnetization whose velocity is encoded, transverse magnetization of a stationary tissue is shifted to longitudinal magnetization by using a 90.degree. pulse called a z-storage pulse. Consequently, only fluid information is extracted, and a velocity image is obtained.
Unfortunately, these conventional methods discussed above have the following problems.
That is, the former method necessitates a double photographing time in order to run the two different pulse sequences, FIGS. 1A and 1B. Additionally, a motor artifact readily takes place due to the time difference between the two pulse sequences. Also, signal processing such as echo data subtraction is complicated.
In the latter method, on the other hand, the S/N ratio is low since only the stimulated echoes contribute to imaging. Also, multi-slice photographing is impossible.